Electrolyte for electroplating metal with tin



Patented June 2, 1925.

UNITED STATES FRANK G. MATHERS, OF BLOOMINGTON, INDIANA.

ELECTROLYTE FOR ELECTROPLATING METAL WITH TIN.

No Drawing.

7 '0 (ll-Z whom it may concern:

Be it known that I, FRANK C. MA'rHnns, a citizen of the United States, residing at Bloomington, county of Monroe, State of Indiana, have invented a certain new and useful Improvement in Electrolyte for Electroplating Metal with Tin, of which the following is a full, clear, and exact description.

My invention relates to the electroplating of sheets of metal or other metallic objects with tin and has for its object to deposit electrolytically a dense, thick coating of finely crystalline structure, free from projections and of a substantially uniform thickness.

A further object of my invention is to provide a process in which a thick coating of such a character can be produced at a rapid rate and in a relatively short time.

By thick coating I mean a coating of a thickness equal to, or over, one thousandth (.001)'of an inch.

In a patent granted to me on November 15, 1921, No. 1,397,222, I have described a process of electrolytic refining of tin, in which the anode was composed of a mixture of tin and lead and in which the electrolyte used contained hydrofluosilicic acid, or its equivalent, together'with sulphuric acid and a small amount of cresylic acid and glue. In carrying out that process the current was passed from the anode to the cathode, resulting in the dissolving of the anode and the depositing of the tin from the solution of the electrolyte upon the cathode, the lead combining with the sulphuric acid and adhering to the anode as an insoluble substance. The deposit made upon the cathode by'the process of that patent with its three-tenths per cent (0.3%) of cresylic acid, except when such deposit is formed slowly by the use of a comparatively small current, is in the form of a coarse, crystalline, solid deposit of substantially uniform thickness, having a rough and an ununiform surface but strongly coherent. Such adeposit is all that is necessary for refining purposes.

I have discovered that cresylic acid when used in a tin plating bath, under proper conditions, enables me to attain results which have long been desired and sought for without success. When added to a tin sulphate bath, for instance, or to the bath de scribed in U. S. Patent No. 916,155 anted to H. L. Hollis, March 23, 1909, it enables me to obtain a much thicker coating than Application filed March 29, 1923. Serial No. 628,433.

responds to a specific gravity 1.158, to which I add cresylic acid, the amount of cresylic acid being such as to make a substantially saturated solution. I also preferably add about .7% by weight of animal glue.

The density of the tin sulphate can be varied through Wide limits and still give good deposits. The rate of deposit should be decreased, however, as the density is lessened. It is important, however, in order to obtain the highest grade deposit to maintain the cresylic acid near the saturation point. For a fair deposit at a slower rate, half saturation by cresylic acid is permissible.

As is Well known, free acid must be present in an aqueous solution of tin sulphate or other tin salts in order to prevent hydrolysis, which-would take place if the solution were neutral. This condition is preferably obtained by using an excess of sulphuric acid. The cresylic acid in my electroplating process functions as an addition agent to prevent the formation of coarsely crystalline cathode deposits, and not to prevent hydrolysis. Cresylic acid is not, properly speaking, an acid and does not practically prevent hydrolysis, if at all.

The tin salt I use comprises tin in the stannous condition combined with at least one, by which I mean one or more, of the acid forming elements of series 2 and 3 of Mendeleefs Periodic Table and included in Groups III, IV, VI and VII thereof, and having atomic-weights of less than 33, the preferred salt being tin sulphate. Mendeleefs Table is to be found on pages 50 and 51 of A Treatise on Chemistry by Roscoe and Schlorlemmer, Vol. 2, published by Macmillan & Co., 1913.

The object to be plated is placed in the bath so as to form a cathode and the plating current applied. A current of about 16 amperes per square foot of surface at a voltage of 1 volts secures proper results, although with a higher voltage and more current the time for electroplating may be reduced. The object while within the bath is preferably or 'forei kept in motion to detach hydrogen bubbles substances suspended in the bath, the motion, when a sheet is being plated, being in the plane of the sheet. v Annealed sheets, on account of the smoothness of coating, can be finished by so few passes through the rolls that the will still be soft. If hard copper is desire the sheets may be rolled so as to harden them either before or after plating. g

In the case of continuous lengths of strips or wire, the continuous-lengths are passed slowly through an elongated bath, proper electrical connections being maintained,

, required.

The expression cresylic acid as used herein is used broadly to mean the more complex oily phenol-like bodies which are somewhat soluble inwater. I preferably use the cresylic acid, knownas cresol, in its crude commercial form. 7

With the current and voltage and the elec trolyte above referred to, a coating of a thickness of two thousandths (.002) of an inch can be obtained in about an hour and other thicknesses in proportionate periods. The coating thus formed is smooth, um-

formly dense,'of a fine crystalline structure, free from projections and of substantially uniform thickness. If a sheet thus coated ispassed through rolls a few times it can be easily bufi'ed so as to take a very high polish. If finished sheets of comparatively soft tity to cause an electrolyte deposit of tin from the solution to be finely crystalline and smooth.

2. An electrolyte for electroplating with tin comprising an acid plating solution of a soluble tin salt comprising tin in stannous condition combined withat least one acid forming element of series 2 and 3 of Mendeleefs Periodic Table and included in Groups III, IV, VI and VII thereof, and

having an atomic weight of less than 33, ap-

proximately saturated with cresylic acid.

3. An electrolyte for electroplating with tin composed essentially of a-solution of tin sulphate approximately saturated with cresylic acid. a

4. An electrolyte for electroplating with tin comprising an acid plating solution of a soluble tin salt of approximately 20 Baum comprising tin in stannous condition combined with at least one acid forming element of series 2 and 3 of Mendeleefs Periodic Table and included, in Groups III, IV, VI and VII thereof, and having an atomic weight of less than 33, and cresylic acid;

5. An electrolyte as characterized in claim 1, with the addition of glue.

6. An electrolyte ascharacterized in claim 2 with the addition of glue.

7. An electrolyte as characterized in claim 3 with the addition of glue.

8. An-electrolyte as characterized in claim 4 with the addition of lue.

v vFRAQN G. MATHERS. 

